Fine particles in the form of powder, 20 microns or less, are used as additives and fillers in a wide variety of compositions such as polymeric and coating compositions. The final properties of the dried or cured composition as well as the ease of dispersion and the amount of filler which may be incorporated into a particular composition are controlled in large measure by the surface properties and particle size of the particular fillers employed. Many small particle size fillers, although possessing suitable properties for particular applications, cannot be used for those applications because they are difficult to disperse, yield highly variable products, can only be incorporated into binders at low pigment volume concentrations, or upon storage in the case of coating compositions, tend to settle and agglomerate rendering the coating compositions useless.
Treating of fine particles with chemicals such as surface active agents to affect the particles' surface properties is not always effective to overcome the above problems. For example, the treating of a filler with an ionic surface active agent to be used in an insulative composition may adversely affect the electrical properties of the composition so it is no longer suitable as an insulation.
Thus, for many applications, it is desirable to alter the surface properties of a particle by coating or encapsulating it in a layer of another material, such as a polymer. If properly coated, the encapsulated particle will exhibit the surface properties of the encapsulating material, such properties often resulting in better wetting properties, ease of dispersion and compatibility with the binder of the polymer or coating composition. Also, for some applications, it is desirable to render the particles readily dispersible in a particular binder yet chemically incompatible with that binder. By coating the surface of the particles with a specific polymer, the final properties of the composition into which the particles are dispersed or blended can be controlled. For example, in the formulation of conductive compositions, fine particle size carbons may be coated with a high polymer which permits the particles to be uniformly dispersed in a plastic, yet be of such a nature as to be incompatible with that plastic. Thus upon annealing, the uniformly dispersed coated conductive particles will be free to reorganize within the plastic matrix so as to form a conductive path. On the other hand, if reinforcement without appreciable conductivity is a desired, the polymeric coating should be one which renders the carbon particle both easily dispersible and compatible with the plastic. Such a coating will cause each individual particle to be completely encapsulated in the plastic matrix, with little agglomeration, resulting in particles completely insulated from one another and with maximum surface area in contact with the plastic for physical reinforcement.
Encapsulation of powders with polymer compositions have, to date, generally been limited to large particle size materials, in the order of 50 microns or greater. See for example Fluid Flow Analysis by G. Sharpe, American Elsevier Pub. Co., Inc., N.Y. 1967 at 370; Drying of Solids in the Chemical Industry by G. Nonhebel and A. Moss, CRC Press, Cleveland, Ohio (N.D.) at 211; Unit Operations of Chemical Engineering by W. McCabe and J. Smith, McGraw-Hill Book Co., N.Y. (N.D. 2nd ed.) at 175; and, Principles of Unit Operations by A. Foust et al, Dept. of Chem. Eng., Lehigh University, Bethlehem, Pa. (N.D.) at 480. Such particles may be encapsulated by fluidized bed coating techniques in which the particles are of sufficient size to be held in suspension by an upwardly flowing gas to form a bed of gas-suspended discrete particles. The polymer coating is then atomized into the chamber, coating the particles. Such a process cannot be operated successfully with fine powders. These powders tend to pack together forming large agglomerates, and thus require an excessive gas flow to fluidize the agglomerates in the bed. The excessive gas flow will transport the fine particles out of the chamber.
Attempts have been made to overcome the difficulties with find particles so they may be coated with low visclosity liquids in a fluidized bed. For example in U.S. Pat. No. 3,237,596, an enclosed fluidized bed apparatus is provided which ejects the particles upward, under pressure, through a nozzle so as to reduce agglomeration, at which time they are coated with an atomized liquid sprayed downward, after which the coated with an atomized liquid sprayed downward, after which the coated particles settle into the fluidized bed. Fine particles carried away by the gas flow are caught in filters and recycled to the chamber. Such a technique, although suitable for coating low viscosity liquids onto particles, has been found impractical for the coating of high molecular weight polymers. Another example of efforts to recycle the fine powder carried away by the gas stream is found in U.S. Pat. No. 3,110,626. This apparatus is also unsuited for coating with high molecular weight polymers, for the reason that there is no means provided for frequent high velocity impact of the particles.
In order to ensure that the final product will be only discrete, coated particles, we have found it necessary to cycle the particles so as to cause frequent high velocity impacts between particles and coating, a process which is not possible by the fluidized bed technique. The repeated impacts are in contrast to other high velocity, impact processes. For example, in U.S. Pat. No. 3,009,826, dispersion and coating of agglomerated particles is accomplished by propelling a stream of liquid and solid at supersonic velocities against a barrier causing the solid to disperse and be coated by the liquid. Although such a process recognizes the need for propelling the particles at high speeds, it makes no provision for recycling the particles, thereby relying on a "one-shot" impact for dispersion and coating. More importantly, such high velocity impact against a solid barrier is inapplicable for the coating of high polymers onto particles. The high viscosities and low flow properties of high polymers make such a technique inapplicable since it is difficult or impossible to get a uniform coating in this manner and the polymer-particle mixture will tend to coalesce and agglomerate on impact with the barrier as opposed to separating as dry coated, discrete particles.
Other known methods for preparing polymer coated powders include pan-coating processes, U.S. Pat. No. 3,711,319 and micro encapsulation, as generally described in U.S. Pat. No. 2,800,457. These processes suffer from the similar disadvantage that they are inapplicable for the coating of finely divided particulate matter in the order of 20 microns or less.
The use of a fluid energy mill type apparatus as generally disclosed in U.S. Pat. No. 3,491,954 provides the capability of impacting agglomerated particles and coating at supersonic speeds, continuously recycling the particles at these speeds so as to prevent reagglomeration and separating out only particles of fine partcle size less than 20 microns. Such mills have in the past been used for the grinding, mixing or blending of fine particles. Also, by maintaining the mill at a temperature below the melt temperature of a polymeric material, such mills have been used for the grinding and blending of high polymer particles. However, it was previously thought that the coating of discrete particles with a high polymer in such a mill was impossible, since unlike the grinding of polymeric materials, it was necessary to cause the polymer to melt and flow about each individual particle. It was believed that the high polymer, upon melting, was so highly viscous, that it would not coat each discrete particle, but rather, cause reagglomeration of the particles with the viscous polymer, especially when applied in appreciable quantities. This has been a problem which has plagued other prior art techniques, and it was thought that the particle-polymer agglomerate would quickly clog the mill and deposit on the sides of the chamber. For this reason, the use of fluid energy mills to coat discrete particles has been limited to liquid or molten low molecular weight materials, used in sufficiently small quantities that they would not cause reagglomeration or coalescence upon the walls of the chamber. Previously, use of a fluid energy mill to coat free-flowing powders has been limited to coating with compositions such as fatty acids, other surface active agents or waxy materials. An apparatus particularly suited for coating particles with low molecular weight materials is discussed in U.S. Pat. No. 3,550,868.
We have now discovered that the use of an apparatus which circulates particles at about supersonic speeds may be used to coat high polymers on small discrete particles, as was heretofore thought impossible.
One application for finely divided coated particulate material in which non-coated particles suffer from all the problems previously discussed, including difficulty of dispersion, agglomeration, and storage stability, are compositions containing carbon blacks. Carbon particles are very fine, sub-micron in diameter. Carbon particles have found wide application in the formulation of conductive compositions in which they are incorporated at high concentrations into a nonconductive binder to render the composition conductive. Such carbon particles generally have high oil absorption characteristics, are difficult to disperse, and readily agglomerate and settle if dispersed in a fluid binder. On account of these characteristics, the amount of carbon which may be dispersed in a particular formulation is generally limited, which in turn limits the conductivity which can be obtained for such coatings. In addition, since many of the carbon or other conductive particles are inadequately dispersed, or have agglomerated upon standing, the conductivity of the final coating is highly variable and irreproducible. Thus, it is apparent that there is a need for conductive particles with predictable and constant electrical properties when dispersed in a binder composition.
As previously discussed, in order to achieve conductive compositions with predictable and constant electrical properties, particles with very specific surface characteristics should be utilized. Generally, the particles should be readily dispersible in the binder to result in a uniform composition, yet be sufficiently agglomerated so as to provide a uniform conductive path throughout the composition.
It is thus an object of this invention to provide fine particles of less than 20 microns having a high polymer coating thereon, such coated particles being free-flowing, non-agglomerative and readily dispersible in a coating or polymeric composition including dry plastics. Selection of the polymer coating will be dictated by the material into which it is to be dispersed, compatibility or non-compatibility of the coating with the binder composition being a major criteria.
It is another object of this invention to provide a high polymer coating on conductive particles of less than 20 microns, such that the particles are free-flowing and readily dispersible, but yet not so encapsulated as to render the particles nonconductive.
It is another object of this invention to provide conductive coating compositions which may be filled with finely divided conductive particles at higher levels than were previously obtainable.